Acousto-optical light deflector having increased band width and short access time

ABSTRACT

An acousto-optical light deflector employs a crystal as a sound medium which is energized by way of a piezo-electric trandsducer with ultrasonic waves to deflect a light beam incident approximately parallel with the sound wave fronts as a function of the ultrasonic frequency. The deflector also comprises a control apparatus for supplying the piezo-electric transducer with a controllable variable frequency and is charactrized by the provision of several piezo-electric transducers which are designed for consecutive frequency ranges and which are juxtaposed on the sound medium.

United States Eschler ACOUSTO-OPTICAL LIGHT DEFLECTOR HAVING INCREASEDBAND WIDTH AND SHORT ACCESS TIME Sept. 18, 1973 11/1971 Pennington etal..... 350/161 7/1956 Arenberg 350/161 Primary Examiner-Ronald L.Wibert Assistant Examiner-V. P. McGraw Attorney-Carlton Hill et al.

[ 5 7] ABSTRACT An acousto-optical light deflector employs a crystal asa sound medium which is energized by way of a piezoelectric trandsducerwith ultrasonic waves to deflect a light beam incident approximatelyparallel with the sound wave fronts as a function of the ultrasonicfrequency. The deflector also comprises a control apparatus forsupplying the piezo-electric transducer with a controllable variablefrequency and is charactrized by the provision of several piezo-electrictransducers which are designed for consecutive frequency ranges andwhich are juxtaposed on the sound medium.

10 Claims, 5 Drawing Figures 3/1972 Anderson et a1. 350/161 PATEHTEB SEP1 8 m3 sum 1 OF 2 Inlllllll l. l

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INVENTOR Han 5 f5 6/) /er BY ACOUSTO-OPTICAL LIGHT DEFLECTOR HAVINGINCREASED BAND WIDTH AND SHORT ACCESS TIME BACKGROUND OF THEINVENTION 1. Field of the Invention This invention relates to a lightdeflector having several transducers for consecutive frequency rangesarranged in juxtaposition, and further relates to an acousto-opticallight deflector comprising a crystal which is utilized as a sound mediumand which is energized by way of a piezo-electric converter withultrasonic waves to deflect a light ray incident approximately parallelwith the wave fronts as a function of the ultrasonic frequency, and alsoto control apparatus for supplying the piezo-electric transducer with acontrollable variable frequency.

2. Description of the Prior Art The principle of acousto-optical lightdeflection has been known for a long period of time. It is based on thelight diffraction by ultrasonic waves. When an ultrasonic wave istransmitted through a medium, for example, a crystal, whereby pressurefluctuations are produced in the crystal, a light ray incident in thedirection of the wave front is diffracted in a manner similar to thataccomplished by a diffraction grating.

The angle of diffraction therefore depends on the distance of thepressure maxima, that is, however, on the wave length and/or thefrequency of the ultrasonic wave. If the direction of incidence of thelight ray against the wave front is inclined by a small angle, a Braggreflection of the light ray can be observed at the wave fronts. In orderfor a Bragg reflection to occur, however, the angle of incidence mustsuffice for the Bragg condition. This principle and its advantages, aswell as various applications were described in 1966 by E. I. Gordon inthe article A Review of Acousto- Optical Deflection and ModulationDevices in the publication Applied Optics, Vol. 5, No. 10, page 1629 etseq., October 1966.

In the article Television Display Using Acoustic Deflection andModulation of Coherent Light by A. Korpel, R. Adler, P. Desmares and W.Watson and published in Applied Optics, Vol. 5, No. 10, page 1667 etseq., October 1966, the authors describe how a larger number ofdeflection directions can be obtained. As it is known, the Braggreflection requires the acoustical wave fronts to be symmetrical withrespect to the incident and diffracted light ray. If the Bragg angle isto be modified, the acoustic wave front must change its direction. Thisis accomplished by a special arrangement and electronic circuitry of thephased array, whose combined wave fronts change their direction when thefrequency is modified. In that way, a change of the ultrasonic frequencyfrom 19 to 35 MHz and a light ray deflection changing in proportionthereto is attainable.

In the periodical Japan J. Apl. Phys. 8," page 81 l, 1969, N. Uchida andH. Ivaski report on an additional structure regarding a two-dimensionalacoustooptical light deflector wherein a sound frequency modificationwas achieved between 48 and 63 MHz through the utilization of a specialdesign.

The frequency bandwidth and the so-called capacity speed product (CSP)connected therewith of acoustooptical light deflectors (the number ofresolvable spots per switching time) are limited by the varying sonicradiation output of the piezoelectric transducers at dif- 2 ferentfrequencies and the solid direction of incidence (Bragg condition).Therefore, the bandwidth of the known acousto-optical light deflectorshas been limited to a maximum of about one octave.

Acousto-optical light deflectors are utilized where a rapid lightdeflection is important. In order to scan a major surface with a lightray, perhaps for profile measurement, the deflectability of the lightray should be provided up to large angles of deflection, whichcorresponds to an effective approachability of the deflection crystalhaving a large bandwidth.

SUMMARY OF THE INVENTION The primary object of the present invention isto provide an increase in the number of deflection angles and toincrease for that purpose the bandwidth of an acousto-optical deflectorto a range encompassing more than one octave.

According to the invention, the foregoing objective is achieved throughthe provision of several piezoelectric transducers which are designedfor consecutive frequency ranges and juxtaposed on a sound medium, thetransducers increasing the sound frequency bandwidth to values up to 300MHZ and maintaining the deflection efficiency effectively constant overa larger range than heretofore known.

The crystal plates used as piezo-electric transducers are preferablyarranged at an angle on the sound medium so that the deflection of theincident light ray can remain stable.

The piezo-electric transducers may also be advantageously arranged in apartial area of a phased array, that is, instead of being tilted withrespect to each other, they may be arranged juxtaposed and parallel andelectrically series connected to a variable oscillator.

It is advantageous to construct the transducers from such differentcrystal materials so that the amplitudes radiated by the transducers arealigned with each other.

Moreover, the transducers are preferably dimensioned in such a mannerthat the frequencies coincide, for which in case of two adjacentfrequencies, the deflection efficiency drops to half the maximum value.The degree of deflection effect is then actually constant over a majorfrequency range.

A control apparatus may advantageously be provided with an oscillator ofa fixed frequency and with an oscillator of variable frequency, as wellas with a mixing device in which the fixed and the variable frequenciesare superposed in order to obtain the control frequencies. Anotherpossibility resides in the control apparatus being provided with anumber of oscillators whose frequencies are invariable and connected byway of electronic switches and a bus bar arrangement with one or severaltransducers. In order to avoid interfering higher waves, the entirefrequency range is advantageously divided into octaves by supplying thefrequencies of different octaves to separate bus bars which are providedwith low pass filters.

The transducers should be connected with the crystal utilized as a sonicmedium, preferably by cold pressing, in vacuum, under the interpositionof compounds having a low melting point, such as indium, thalium, etc.

BRIEF DESCRIPTION OF THE DRAWINGS Other objects, features and advantagesof the invention, its organization, construction and operation will bestbe understood from the following detailed description taken inconjunction with the accompanying drawings, in which:

FIG. I is a pictorial and schematic representation of an acousto-opticallight deflector having increased bandwidth;

FIG. 2 is a graphical illustration of the total deflection efficiencyeffect of the multiple transducer deflector;

FIG. 3 illustrates a phased array design of an acoustooptical deflector;

FIG. 4 is a block diagram illustration of an oscillator which issynchronizable throughout the entire frequency range; and

FIG. 5 is a block diagram representation for a digitalized control ofthe transducers.

DESCRIPTION OF THE PREFERRED EMBODIMENTS In FIG. 1, a deflector crystal1 is illustrated as a sound medium. The deflector crystal 1 carriesthree acoustic transducers 2, 3 and 4 with different frequency ranges ofthe sound radiation, namely, fl to f2, f2 to f3, and f3 to f4. The threetransducers 2, 3 and 4 may be connected in parallel and connected to ahigh frequency oscillator 5. Each transducer actually represents aband-pass filter and accommodates an electrical output only in itscorresponding frequency band. The transducers deliver the absorbedenergy to the deflection crystal in the form of ultrasonic waves,whereby compressions 6 are produced in the deflection crystal. Thedistance of the illustrated compression lines corresponds to the wavelengths of the ultrasonic waves. If a laser beam enters the deflectioncrystal 1 from a fixed predetermined direction, it is deflected at thewave fronts 6 according to the Bragg condition. The incident laser beam7 is deflected, depending on the ultrasonic frequency present, into thedirection 8, 9 or 10. The number of possible directions of deflection ofthe laser beam is a function of the number of acousto-opticaltransducers and their usable frequency bandwidths.

It may also be advantageous to traverse the light beam as rapidly aspossible over different directions of deflection. The scale for theefficiency of the light deflector is the so-called capacity speedproduct CSP. It is only a function of the bandwidth of the deflector andthe expression Af/2 applied. With the structure described, bandwidthsbetween lOO and 300 MHz and capacity speed products of about 2 X 10'seconds are possible.

FIG. 2 is a graphical illustration of the degree of deflection effect 1;with respect to the frequency f. The degree of effect is understood tomean the relation of deflected to incidence luminous intensity. FIG. 2illustrates that each of the three transducers 2, 3 and 4 has arespective degree of effect of 1) l, n 2 and/or 7 3, which has a maximumvalue at the corresponding central frequencies fl, f2 and f3. Thedegrees of effect drop off on both sides of these central frequencies.The central frequencies fl, f2 and [3 are spaced such that the degreesof effect bisect where they have dropped by three decibels. From thesethree curves, a total degree of effect of n, 1; l 17 2 1 3, asrepresented in the drawing. The degree of effect of a transducer is afunction of the data of the sound medium, the light wavelength of theincident beam, the dimensions of the transducer and the sound or sonicperformance. At suf- 4 ficiently high sonic performance, percent of theradiated light can be deflected, because no performance is lost underthe alternating effect of the light waves with the sonic field.

The manner of operation of a phased array design is illustrated in FIG.3. Here again, the crystal 1 is employed as a sonic medium and carriesthree transducers 2, 3 and 4 which are juxtaposed parallel with eachother. The transducers are electrically seriesconnected with a variableoscillator 5, so that at a time when the transducer 3 causes at acertain distance a compression 36 in the crystal, compressions 37 and 38shifted by A /2 are generated by the transducers 2 and 4. Thecompressions 36, 37 and 38 can be consolidated into a single compressionline 39 extending obliquely in the crystal. In this way compressionsextending obliquely and shifted by A are produced in the crystal, whoseoblique position depends on the oscillator frequency.

FIG. 4 illustrates how a variable control of the transducers is madepossible with a fixed and a variable oscillator. In FIG. 4, anoscillator 10 has a fixed frequency fa and a variable oscillator l 1provides frequencies of fb and fa, whereby the frequency fb is greaterthan the frequency fa. Both frequencies are superposed in a mixer 12 andsupplied to the transducers by way of a low pass filter 13 forfrequencies which are less than fc-fa and by way of a broad bandamplifier 14. The low pass filter 13 is utilized to eliminate the upperfrequency waves from influencing the deflection of the luminous beam.

FIG. 5 illustrates a block circuit diagram for a digitalized approachfor energizing the transducers. Here, the oscillators 15, 16 and 17 haveseparate fixed frequenciesfl5,fl6 and fl7, respectively, and areconsolidated by way of a bus bar 22 as a first group of oscillators, andthe oscillators 18, 19, 20 and 21 have fixed frequencies fl8,f19, j20and 121, respectively, which are consolidated by means of a bus bar 23as a second group of oscillators. One frequency octave is contained ineach group. A desired frequency can be switched to a bus bar from one ofthe switching inputs 24 or 25 by way of an appropriate switch gate 26.Therefore, higher waves are created which are prevented from traversingthe low pass filters 27 and 28. The selected frequency fl5,fl6 or f21passes to the transducers by way of a wide band amplifier 29.

The foregoing has described acousto-optical light deflectors and meansfor operating such deflectors with increased bandwidth. Although theforegoing description has been made by reference to certain illustrativeembodiments, many changes and modifications thereof may becomeapparentto those skilled in the art without departing from the spirit and scopeof my invention. Therefore, it will be appreciated that I'intend toinclude within the patent warranted hereon all such changes andmodifications as may reasonably and properly be included within thescope of my contribution to the art.

I claim:

1. An acousto-optical light deflector comprising a crystal employed as asonic medium, a plurality of piezoelectric transducers carried on saidcrystal, said piezo-electric transducers comprising crystal platescarried on said crystal sound medium and angularly disposed with respectto one another, each of said transducers having an individual frequencyrange adjacent to the frequency ranges of the other said transducers andenergizable with ultrasonic energy to produce sonic wave fronts in saidcrystal as a function of the energizing frequency for deflecting a lightbeam incident approximately parallel with said sonic wave fronts, andcontrol apparatus for supplying said piezo-electric transducers with acontrollable variable frequency.

2. An acousto-optical light deflector according to claim 1, wherein saidpiezo-electric transducers are carried by said crystal juxtaposed andparallel and are electrically connected to each other and to saidcontrol apparatus.

3. An acousto-optical light deflector according to claim 1, wherein saidpiezo-electric transducers are constructed from different crystalmaterials so that the sonic amplitudes radiated by the transducers arealigned with each other.

4. An acousto-optical light deflector according to claim 1, wherein saidtransducers are constructed such that the frequencies of adjacenttransducers coincide in partial frequency ranges below their half powerpoints.

5. An acousto-optical light deflector according to claim 1, wherein saidcontrol apparatus includes a fixed frequency oscillator, a variablefrequency oscillator and a mixer connected between said oscillators andsaid transducers for superposing said fixed and variable frequencies toprovide a control frequency.

6. An acousto-optical light deflector according to claim 1, wherein saidcontrol apparatus includes a plurality of fixed frequency oscillatorseach having a different frequency, a bus bar connected to saidtransducers, and a plurality of electronic switches connected betweensaid oscillators and said bus bar for selectively connecting saidoscillators to said transducers.

7. An acousto optical light deflector according to claim 6, wherein thetotal frequency range is divided into octaves and said bus bar isprovided as a plurality of buses each associated with a separate octave,and a plurality of low pass filters each interposed between a separatebus and said transducers.

8. An acousto-optical light deflector according to claim 1, wherein saidtransducers are connected with said crystal by cold pressing in a vacuumand comprising a low melting point compound interposed between saidtransducers and said crystal.

9. An acousto-optical light deflector according to claim 8, wherein saidlow melting compound comprises indium.

10. An acousto-optical light deflector according to claim 8, whereinsaid low melting compound comprises thalium.

1. An acousto-optical light deflector comprising a crystal employed as asonic medium, a plurality of piezo-electric transducers carried on saidcrystal, said piezo-electric transducers comprising crystal platescarried on said crystal sound medium and angularly disposed with respectto one another, each of said transducers having an individual frequencyrange adjacent to the frequency ranges of the other said transducers andenergizable with ultrasonic energy to produce sonic wave fronts in saidcrystal as a function of the energizing frequency for deflecting a lightbeam incident approximately parallel with said sonic wave fronts, andcontrol apparatus for supplying said piezo-electric transducers with acontrollable variable frequency.
 2. An acousto-optical light deflectoraccording to claim 1, wherein said piezo-electric transducers arecarried by said crystal juxtaposed and parallel and are electricallyconnected to each other and to said control apparatus.
 3. Anacousto-optical light deflector according to claim 1, wherein saidpiezo-electric transducers are constructed from different crystalmaterials so that the sonic amplitudes radiated by the transducers arealigned with each other.
 4. An acousto-optical light deflector accordingto claim 1, wherein said transducers are constructed such that thefrequencies of adjacent transducers coincide in partial frequency rangesbelow their half power points.
 5. An acousto-optical light deflectorAccording to claim 1, wherein said control apparatus includes a fixedfrequency oscillator, a variable frequency oscillator and a mixerconnected between said oscillators and said transducers for superposingsaid fixed and variable frequencies to provide a control frequency. 6.An acousto-optical light deflector according to claim 1, wherein saidcontrol apparatus includes a plurality of fixed frequency oscillatorseach having a different frequency, a bus bar connected to saidtransducers, and a plurality of electronic switches connected betweensaid oscillators and said bus bar for selectively connecting saidoscillators to said transducers.
 7. An acousto-optical light deflectoraccording to claim 6, wherein the total frequency range is divided intooctaves and said bus bar is provided as a plurality of buses eachassociated with a separate octave, and a plurality of low pass filterseach interposed between a separate bus and said transducers.
 8. Anacousto-optical light deflector according to claim 1, wherein saidtransducers are connected with said crystal by cold pressing in a vacuumand comprising a low melting point compound interposed between saidtransducers and said crystal.
 9. An acousto-optical light deflectoraccording to claim 8, wherein said low melting compound comprisesindium.
 10. An acousto-optical light deflector according to claim 8,wherein said low melting compound comprises thalium.